From: luto@amacapital.net   
      
   I have a class template that should be, depending on its parameters,   
   nontrivially copyable, trivially copyable, or not copyable.  It looks   
   like this:   
      
   template   
   struct A   
   {   
   	A(const A&) { body here }   
      
   	// Members that contain one instance of each type in Types.   
   };   
      
   If the body is ill-formed, all is well -- A can't be copied.  If all of   
   the types are copyable and at least one doesn't have a trivial copy   
   constructor, all is still well: A is copyable.   
      
   I want A to be trivially copyable, though, if all the types are   
   trivially copyable.  The best I've come up with is:   
      
   template   
   struct helper   
   {   
   	// if all types are trivially copyable, then   
   	typedef garbage type;   
   	// else   
   	typedef A type;   
   };   
      
   template   
   struct A   
   {   
   	A(const typename helper::type &) { body here }   
      
   	// Members that contain one instance of each type in Types.   
   };   
      
   This works until I add a move constructor.  Then I end up with:   
      
   template   
   struct A   
   {   
   	A(const typename helper::type &) { body here }   
   	A(A&&) { ... }   
      
   	// Members that contain one instance of each type in Types.   
   };   
      
   If the "copy" constructor is a real copy constructor (i.e. helper::type   
   is A), then it works.  If not, though, then this is equivalent to:   
      
      
   template   
   struct A   
   {   
   	A(const garbage &) { body here }   
   	A(A&&) { ... }   
      
   	// Members that contain one instance of each type in Types.   
   };   
      
   which is not copyable at all.  What I really want is:   
      
   template   
   struct A   
   {   
   	A(const typename helper::type &) { body here }   
   	A(const A &) = default;  // But only sometimes   
   	A(A&&) { ... }   
      
   	// Members that contain one instance of each type in Types.   
   };   
      
   which is ill-formed if helper::type is A.   
      
      
   Any ideas?  The default template argument SFINAE trick doesn't work   
   because copy constructors can't be templates.   
      
   Thanks,   
   Andy   
      
      
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